<!DOCTYPE html>
<html lang='en'>
<head>
  <meta charset='UTF-8'>
  <meta name='viewport' content='width=device-width, initial-scale=1.0'>
  <title></title>
  <script type='text/javascript' src='js/vue.js'></script>
</head>
<body>
  <div id='app'>
    <div>
      <input type="text" v-model="aa">
      <input type="text" v-model="bb">
      <p>{{cc}}</p>
    </div>
  </div>
  <script type='text/javascript'>
    var vm = new Vue({
      el: '#app',
      data: {
        aa: 'Hello',
        bb: 'World',
        cc: ''
      },
      methods: {
        
      },
      watch: {
        aa(val){
          this.cc = val + this.bb;
        },
        bb(val) {
          this.cc = val + this.aa;
        }
      },
    });
  </script>
</body>
</html>